nuckolls.bob(at)aeroelect
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 Posted: Sun Jan 31, 2010 5:59 pm Post subject: Using a SPST switch to control both the alternator field a |
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At 04:41 PM 1/31/2010, you wrote:
List,
I have a switch that I really like that I want to use for the main
switch. It's a heavy duty switch that can handle up to 75amp, but it's
only a SPST switch.
What kind of airplane/engine? Where is your battery located
with respect to the proposed location for the battery master
switch?
The diagrams in the Aeroelectric connection show a DPST switch for the
master because it turns off the alternator field coil. This leads me to
ask two questions:
1. Does the field coil need to be turned off?
It's customary in type certificated aircraft to have one
switch that controls DC power for all sources. I.e., a MASTER
switch.
If it's wired directly to the main power bus then the second the
master switch is turned off the contactor isolates the main buss
anyway causing the field coil to shut down.
No. Alternators can . . . and often do continue to run
"self-excited" unless you break the field supply lead
(or open the ON/OFF command lead for internally regulated
alternators).
2. If it's harmful to have the field coil wired to the main power buss
without a way to isolate it, then can I wire my main power switch as
shown in the attached drawing? Other than a little more wire, is there
anything wrong with doing it this way?
I didn't see an drawing come through as an attachment.
If it's your intention to replace a battery contactor with
a manually operated switch (common to many aircraft back
in the 40's and 50's) then you need a separate alternator
control switch. In this configuration, you don't have a
MASTER switch and both switches need to be opened to kill
the system.
The subject of your note does raise a question . . . do
you plan to use a battery contactor or is the 75A rated
switch intended to be the replacement for a battery
contactor?
Bob . . .
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